1. Field of the Invention
The present invention relates to a discharge lamp lighting method and a discharge lamp lighting apparatus adopted to light a discharge lamp by supplying AC power to the discharge lamp. More specifically, it relates to a discharge lamp lighting method and a discharge lamp lighting apparatus that are ideal in applications in which a high-pressure discharge lamp, a high-pressure mercury lamp or an ultra high-pressure mercury lamp is driven by AC power.
2. Background of the Prior Art
It is a known fact that a high-pressure discharge lamp can be driven with a relatively high degree of efficiency by supplying low-frequency AC power with a frequency of, for instance, approximately 50 to 500 Hz to the high-pressure discharge lamp as disclosed in U.S. Pat. No. 4,485,434 and the like.
However, when lighting a high-pressure discharge lamp by supplying AC power, the position at which the discharge arc is generated on an electrode, which depends upon the electrode temperature at the high-pressure discharge lamp and the state of the electrode surface, becomes unstable, resulting in a problematic phenomenon in which the originating point of the discharge arc at the electrode jumps from a given point to another point. This phenomenon manifests as a flickering of the lamp light (lamp flicker).
Published Japanese Translation of PCT International Publication for Patent Application No. 1998-501919 discloses a means for reducing the extent of such lamp flickering. The invention disclosed in this prior art publication is achieved by generating a current pulse having the same polarity as the lamp current over intervals each corresponding to a half cycle of the lamp current and superimposing the current pulse thus generated on the lamp current during the terminal phase of each half cycle of the lamp current to suppress lamp flickering.
However, in the invention disclosed in the prior art publication mentioned above, a current pulse sustained over a specific length of time must be generated in synchronization with the terminal phase of each half cycle of the lamp current and the current pulse thus generated must be superimposed on the lamp current with specific timing. In addition, it is necessary to set a reference value for generating the current pulse. For these reasons, the circuit for generating the current pulse must achieve a highly advanced time management function, which necessitates a complex circuit structure.
It is an object of the present invention to provide a discharge lamp lighting method and a discharge lamp lighting apparatus achieved through a relatively simple circuit structure, which may be adopted to reduce the extent of lamp flickering.
It is a further object of the present invention to provide a discharge lamp lighting method and a discharge lamp lighting apparatus that contribute to lengthening the service life of a discharge lamp by lessening the extent of electrical shock to the discharge lamp.
In order to achieve the objects described above, in the discharge lamp lighting method according to the present invention, when lighting a discharge lamp by supplying AC power, the AC power is provided as an AC pulse current and an AC pulse voltage, and the discharge lamp is driven by gradually increasing an instantaneous value of the AC power during a half cycle of the AC pulse current over the time elapsing from the rise of the half cycle toward an end of the half cycle.
Since the instantaneous value of the AC power is gradually increased during the half cycle of the AC pulse current supplied to the discharge lamp over the time elapsing from the rise of the half cycle toward an end of the half cycle, the level of the AC power supplied to the discharge lamp reaches the maximum value at the end of each half cycle allowing the temperature of the electrode of the discharge lamp to rise to a high level at the end of the half cycle. This stabilizes the discharge arc and reduces the occurrence of lamp flickering.
The discharge lamp lighting method according to the present invention, in which the instantaneous value of the AC power is gradually increased during the half cycle of the AC pulse current over the time elapsing from the rise of the half cycle toward an end of the half cycle, only requires AC pulse current waveform control to be implemented. It does not necessitate any time management for superimposing the current pulse. As a result, simplification in the circuit structure is achieved. The AC power is controlled so as to achieve the average power level required in the discharge lamp. Thus, the discharge lamp is driven with a high degree of reliability.
In addition, since the instantaneous value of the AC power is gradually increased over the time elapsing from the rise of the half cycle of the AC pulse current toward an end of the half cycle, the extent of the electrical shock to which the discharge lamp is subjected is lessened to contribute to lengthening the service life of the discharge lamp, unlike the prior art technology achieved by superimposing a stepped current pulse on the lamp current.
The present invention also discloses a discharge lamp lighting apparatus employed to implement the discharge lamp lighting method described above. This discharge lamp lighting apparatus comprises a power supply unit and a control unit. The power supply unit supplies AC power to the discharge lamp. The AC power is provided as an AC pulse current and an AC pulse voltage.
The control unit implements control on the power supply unit so as to gradually increase the instantaneous value of the AC power during a half cycle of the AC pulse current over the time elapsing from the rise of the half cycle toward an end of the half cycle.
This discharge lamp lighting apparatus, which clearly may be utilized to implement the discharge lamp lighting method according to the present invention, is capable of stabilizing the discharge arc and reducing the occurrence of lamp flickering. In addition, since it does not require any time management and requires only AC pulse current waveform control, simplification of the circuit structure is achieved.
The present invention further discloses specific circuit structure examples that may be adopted in the discharge lamp lighting apparatus. Other objects, structural features and advantages of the present invention are explained in further detail by referring to the attached drawings. The drawings simply present examples.